Field of the Invention
The present invention relates to a photoconductive device, a measurement apparatus including the photoconductive device, and a method of manufacturing a photoconductive device.
Description of the Related Art
A terahertz wave is an electromagnetic wave having a frequency of no less than 30 GHz and no greater than 30 THz. A photoconductive device can be used as a device for generating or detecting a terahertz wave. A photoconductive device is constituted by a semiconductor material that has relatively high mobility and has a carrier lifetime of no greater than a picosecond, and by antenna electrodes (bias lines) that also serve as electrodes provided on the semiconductor. When a gap portion between the antenna electrodes is irradiated with ultrashort pulsed light, in a state in which a voltage is applied across the antenna electrodes, a current flows instantaneously between the antenna electrodes due to excited optical carriers, and thus the photoconductive device radiates a terahertz wave. In order to carry out accurate measurement of terahertz radiation, by using such a photoconductive device, there is a demand for an increase in the power of a terahertz wave generated by a photoconductive device, an increase in the detection sensitivity of the terahertz wave, and an increase in the generation or detection bandwidth of the terahertz wave.
As a means for increasing the generation power and the detection sensitivity of a photoconductive device, Japanese Patent Application Laid-Open No. 2013-80939 describes a photoconductive device in which a semiconductor layer formed of epitaxially grown GaAs is disposed on a substrate having low terahertz-wave absorptivity with a carrier movement prevention layer disposed between the substrate and the GaAs layer. Disposing the carrier movement prevention layer between the substrate and the GaAs layer makes it possible to reduce the movement of generated excitation carriers toward the substrate and therefore increases the amount of excitation carriers used to generate or detect a terahertz wave.
In addition, U.S. Patent Application Publication No. 2014/0346357 describes a photoconductive device provided with a diffraction grating that produces surface plasmon resonance between electrodes. Being provided with the diffraction grating, the photoconductive device described in U.S. Patent Application Publication No. 2014/0346357 enhances the electric field of excitation light on a semiconductor surface of a gap portion between the antenna electrodes and thus increases the amount of light absorption in the semiconductor. This configuration increases the generation efficiency of optical carriers. Specifically, a periodical structure in the order of submicrons, such as a diffraction grating, is formed between the antenna electrodes by a material having a negative dielectric constant, such as metal. Free electrons in the diffraction grating collectively vibrate upon being irradiated with ultrashort pulsed light, which induces surface plasmon resonance. Characteristic features of the surface plasmon resonance include an electric field enhancement effect and a magnetic field localization effect, which enable an increase in the generation efficiency of optical carriers in the vicinity of an interface between the diffraction grating and the semiconductor. U.S. Patent Application Publication No. 2014/0346357 reports that the power of the generated terahertz wave is higher by ten times or more when a diffraction grating is provided between the antenna electrodes than when the diffraction grating is not provided.